Abstract: First results from an integrated Boltzmann + hydrodynamics hybrid approach

First results from UrQMD-3.0 (a Boltzmann approach with an intermediate hydrodynamic stage) for heavy ion reactions from GSI-SIS to the highest CERN-SPS energies will be presented. Event-by-event fluctuations are directly taken into account via eventwise non-equilibrium initial conditions generated by the initial collisions and string fragmentations in the microscopic UrQMD model. After a (3+1)-dimensional hydrodynamic evolution, the hydrodynamical fields are mapped to hadrons via the Cooper-Frye formula and the subsequent hadronic cascade calculation within UrQMD proceeds to incorporate the important final state effects and a realistic freeze-out. This implementation allows to compare pure transport calculations with hydrodynamic calculations using exactly the same initial conditions and freeze-out. The effects of the change in the underlying dynamics -ideal fluid dynamics vs. non-equilibrium transport- will be explored. The pion and proton multiplicities are surprisingly similar in both cases while the yields for strange particles are enhanced due to the local equilibration in the hydrodynamic evolution. The results of the different calculations for the mean transverse mass and elliptic flow excitation functions will be discussed in the context of the available data. The lower mean free path leads to higher pressure gradients in the early stage and as a consequence to higher elliptic flow values even without phase transition.